Cohesive failure adhesive test

T Urethanes not shown because of great differences in physical properties, depending on formulations. Adhesion characteristics should he related by actual test data. Any system which shows concrete failure when tested for surfacing adhesion should he rated excellent with decreasing rating for systems showing failure in cohesion or adhesion below concrete failure. 

Each of the multilayered materials of Table II, in pouch form, met the retortability requirements. Examination of the pouches after this test showed that no delamination occurred among the layers. However, microscopic examination of specimens used for bond strength tests showed that adhesive failure rather than cohesive failure occurred be-... 

Knife test (KNF) The test is done by making two intersecting scratches through the paint film to the substrate with a sharp steel knife. Adhesive or cohesive failures are evaluated by peeling the coating from the intersection point and outwards. Common for the three adhesion evaluation methods are that the test is performed on immersed and non- immersed panel-half (referred to as respectively "wet" and "dry" adhesion). The type of rupture is reported, and the severity is judged on a scale from 5 (perfect) to 0 (poor). 

With time (under increased temperature and humidity) the crack tip continues to a weaker region which for this surface treatment appears to be near the oxide/alloy interface. Figure 11 summarizes the analysis of the bond failure for this particular surface treatment. The important aspect here is that under identical conditions, different surface preparations show different modes of failure. Weak boundary layers are not developed using some treatment/bonding combinations. Processes have been developed in which the locus of failure remains in the adhesive ("a cohesive failure") and it is necessary to use a mechanical test in which even more stress is placed on the interfacial region (19). 

High peel strength with cohesive failure is possible by increasing the 0K/NC0 ratio. At a higher ratio, slight creep is observed at room temperature in the static shear test., 0H/NC0 ratios above 2.0 for Hycar 2103 and 1.8 for Hycar 2106 result in adhesives having incomplete cures with low peel values and poor static shear strength. 

It must be emphasized that polymer adhesion is a complex phenomenon. The efficiency of an adhesive is only partly determined by interfacial properties. Cassidy et al. (1972) already found that effects on the glass transition temperature of the adhesive may be more important than interfacial properties. An additive that lowers Tg from a point above the test temperature to below it, causes a decrease in the strength of the system with cohesive failure within the adhesive. 

Because of the high scattering of experimental results and the great difficulty in reaching the fully cohesive failure of wooden adhesive Joints, a numerical analysis has been made to give a better knowledge of their mechanical behaviour for various parameters (adhesive used. Joint thickness, loading mode, etc...). For the PU resin tested previously in shear, such an analysis has been made on two steps first simulations have been made on bulk adhesive specimen to determine the mechanical behaviour of the resin and the numerical results obtained have been implanted in the FE code CASTEM 2000 [21] for the mTENF bonded specimen loaded by shear. 

Performing a peel test after lamination should be considered as only a good screening test it is not sufficient for material selection. Plueddemann recommends a 7-day soak in room-temperature water as an additional test, with final peel tests demonstrating cohesive failure rather than adhesive failure (Reference... 

In order to understand the aim of surface treatments for composite bonding, we wiU concentrate on a real composite bonding problem for aeronautical purposes. Since classical surface treatments like peel ply can be limited by a cohesive failure occurring in the material, we wiU focus on a new kind of surface treatment (excimer laser) which can completely change surface parameters. The different aspects are presented in two steps the first consists in the surface characterization of the composite material and the second is related to results of destructive single lap shear tests of composite assemblies. Finally, both steps are Hnked in order to derive general mles on phenomena governing adhesion properties of polymer composites. 

The pull-off test for measuring adhesion is described in ISO4624. Adhesion is assessed by measuring the minimum tensile stress necessary to detach or to rupture the coating in a direction perpendicular to the substrate. The result gives the minimum tensile stress required to break the weakest interface (adhesive failure) or the weakest compound (cohesive failure) of the test assembly. Mixed adhesive-cohesive failure may also occur. 

In his test, a thin film of adhesive on a glass microscope slide or a metal coupon is cured and soaked in hot water until the film can be loosened with a razor blade. There is usually a sharp transition between samples that exhibited cohesive failure in the polymer and those which exhibited more of an interfacial failure. Since the diffusion of water into the interface is very rapid in this test, the time to failure is dependent only on interfacial properties and may differ dramatically between unmodified epoxy bonds and epoxy bonds primed with an appropriate silane coupling agent. The time to debond in the hot water for various silane primers differed by several thousandfold when used with a given epoxy. In parallel tests, a thick film of epoxy adhesive on nonsilaned aluminum coupon showed about the same degree of failure after 2h in 70°C water as a silaned joint exhibited after more than 150 days (3600 h) under the same conditions. 

Here the adhesive is applied under standard conditions to whatever test surface is felt appropriate. The test, usually adhesion or shear, is then carried out in the high temperature environment, after allowing sufficient time for the assembly to reach the test temperature before beginning the test. Visual examination for signs of cohesive failure is a necessary part of high temperature adhesion testing. 

The result shows that the determined values for the tensile adhesion strength are extremely low. The tensile adhesion strengths of all cement stone samples are significantly lower than the tensile strengths of both stones and the cement stones themselves. When judging the fracture pattern it could be observed that above all it had come to a cohesion failure of the stones. The examination of the test pieces before test start showed a pre-damage on all samples which had not been there on the stone slices after demoulding (Fig. 7). 

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